The present disclosure relates to a solid-state imaging apparatus.
As a solid-state imaging apparatus, a CCD (Charge Coupled Device) image sensor and a CMOS (Complementary Metal Oxide Semiconductor) image sensor are known. The solid-state imaging apparatuses just mentioned are widely used as an imaging device for a digital video camera, a digital still camera and a portable telephone set. In recent years, development of the CMOS image sensor whose power consumption is lower than that of the CCD image sensor has been advancing.
As the CMOS image sensor, an image sensor called column-parallel AD conversion type or column ADC (Analog-to-Digital Converter) type is available. The column ADC type image sensor includes a floating diffusion (FD) for each of pixels thereof, and selects one arbitrary row in the pixel array and accesses pixels for one row arranged in a column direction at the same time to read out pixel signals in parallel at the same time. Then, the read out pixel signals are AD converted, and digitalized image data for one column are outputted to a signal processing section.
As such an image sensor as described above, an image sensor is available in which difference processing between a signal level or reset level when the FD is reset and another signal level upon imaging is carried out to extract a pixel signal whose noise component is removed. This image sensor is preferable because a picture quality enhancement effect by noise removal is obtained in a general imaging condition.
Incidentally, it has turned out that, in a certain imaging condition, an adverse effect appears by carrying out such a difference process as described above. In particular, such an adverse effect as just described appears under an imaging condition of a greater light amount than a light amount at a saturation level such as a condition that there is a high luminance object such as the sun, a lamp or the like within an imaging range or the like. This adverse effect is called sunspot phenomenon or tanning phenomenon and is a phenomenon that a portion of a great light amount which is to be originally imaged as high-luminance white is imaged as a low-luminance portion or becomes black.
The phenomenon described above is caused from a phenomenon that, when very intense light is inputted to a charge generation section (photoelectric conversion device) of a pixel, charge generated by the charge generation section leaks out to the FD by capacity coupling and the reset level drops as time passes. Where the variation amount of the reset level is represented by ΔV and the signal level of a pixel signal corresponding to a signal charge amount acquired from the charge generation section is represented by Vb, Vb−ΔV is extracted as the pixel signal of the pixel by difference processing. In order to identify the extracted pixel signal from the pixel signal after the difference processing, a raw pixel signal acquired from a pixel is hereinafter referred to conveniently as “primitive pixel signal.” If strong light is inputted to the charge generation section and the saturation level is reached, then the primitive pixel signal Vb is fixed without dropping from the reached level. On the other hand, the reset level drops to increase the variation amount ΔV. Therefore, the difference Vb−ΔV decreases and the pixel signal to be outputted drops. As a result, a pixel signal drops although strong light is irradiated on a pixel, and as a result, a tanning phenomenon that a high-luminance object becomes a black image occurs.
In order to suppress occurrence of the tanning phenomenon, (1) a method is available in which the reset level is replaced into a predetermined value or the reset level is clipped (for example, refer to Japanese Patent Laid-Open No. 2000-287131). Further, (2) another method has been proposed in which control for an comparator output when a pixel signal level is read out is changed in response to a result of comparison between the reset level and a reference signal (for example, refer to Japanese Patent Laid-Open No. 2007-243266, hereinafter referred to as Patent Document 1). Further, (3) a method has been proposed in which an influence of dispersion of the reset level can be moderated upon decision of a tanning phenomenon (for example, refer to Japanese Patent Laid-Open No. 2008-283557, hereinafter referred to as Patent Document 2).